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Atari 130XE SIO Port Repair

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Atari 130XE (rear)
The rear of a PAL 130XE: the 13-pin peripheral (SIO) socket J1 is at the left, then the cartridge and expansion slots, the monitor socket, the RF output, the power socket and the power switch

Disk drives, cassette recorders, printers and interfaces connect to the Atari 130XE through the 13-pin serial (SIO) socket J1 on the back. On Atari's schematic the serial data and clock lines come from POKEY (U22) through 100 Ω resistors (R128 to R131), and the control lines from the PIA (U23): the command line from CB2, the proceed and interrupt lines to CA1 and CB1, and the cassette motor control from CA2 through the switching transistor Q7 (MPSA55).[1] Sams' service data gives a check for each direction of the serial path and for the motor control, using a cassette recorder.[2]

Symptoms

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  • The computer will not save a program to tape, or to another SIO device.[2]
  • The computer will not load a program from tape.[2]
  • The cassette recorder's motor does not start when saving or loading.[2]
  • Sams' preliminary checks put a cassette motor fault at the PIA (U23), and save or load faults at POKEY (U22).[2]

Check the peripheral and its cable on another computer first, and clean the pins of J1 and the plug.

Safety

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Pin 10 of J1 carries +5 V from the computer to the peripheral.[1] Do not short it to the neighbouring pins while probing a live socket.

Tools and parts

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  • A logic probe and an oscilloscope.[2]
  • A cassette recorder and a tape, for Sams' checks.
  • Replacement parts as found: MPSA55 for Q7 (Atari C014809), 2.7 kΩ for R91, 100 Ω for R128 to R131, 1000 pF filter capacitors (C301 to C310, C070121).[1]

Procedure

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Socket pinout

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SIO socket J1 (Atari schematic C070065)
Pin Signal Pin Signal
1 Clock in 8 Motor control
2 Clock out 9 Proceed
3 Data in 10 +5 V / ready
4 Ground 11 Audio in
5 Data out 12 Not connected
6 Ground 13 Interrupt
7 Command

[1]

Open the machine

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  1. Remove the four screws from the bottom of the case, lift the top and disconnect the keyboard cable.[2]
  2. Remove the seven screws holding the board, lift out the board and shield, and twist the seven tabs to separate the shield halves.[2] Reconnect the keyboard, power and the recorder for the checks.

Will not save

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Atari 130XE POKEY serial lines to SIO J1 (C070065 schematic)
POKEY's serial pins (27, 26, 24 and 28) reach J1 pins 2, 1, 3 and 5 through the 100 Ω resistors R128 to R131 on Atari's schematic
  1. Type a short program, or load one, and save it to tape.[2]
  2. While it saves, check the waveforms on POKEY (U22) pins 26, 27 and 28. If any is missing, check U22 by substituting another POKEY.[2]
  3. If the waveforms are correct, check the connections at J1 pins 1, 2 and 5.[2] On Atari's schematic each of these lines passes through one of the 100 Ω resistors R128 to R131 on its way to J1.[1]

Will not load

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  1. Check for pulses at POKEY (U22) pin 24 while loading a program from tape.[2]
  2. If the pulses are present, check the connections at J1 pin 3.[2]

Motor does not start

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Atari 130XE PIA U23, controller ports and SIO control lines (C070065 schematic)
The PIA U23 and, at the top, the motor-control transistor Q7 (MPSA55) driven from CA2 (pin 39) through R91, with the ferrite beads L10 and L11 to J1 pins 10 and 8
  1. Type POKE 54018,52 and press RETURN. The logic probe on pin 39 of the PIA (U23) should go from high to low, which starts the recorder's motor.[2]
  2. If the reading stays high, check U23 by substituting another PIA.[2]
  3. If the reading changes normally, check the switching transistor Q7 and the connections at J1 pin 8.[2]
  4. Type POKE 54018,60 and press RETURN to turn the motor off again.[2]

Repair

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  1. Resolder cracked joints at J1 and replace any open resistor or bead found in the failing line, or Q7 for a motor fault.
  2. Refit the shield and bend its tabs, refit the board with its seven screws, reconnect the keyboard and refit the top.[3]

Testing

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  1. Save a program to tape and load it back, and check that the motor starts and stops.[2]
  2. If the production test cartridge and its serial loop-back plug are available, the ports test should report PASS. Error 34 points at the serial data or clock lines, POKEY or the loop-back plug; error 35 at the motor control, proceed, interrupt or command line, POKEY or the plug; error 36, when 35 does not also occur, at POKEY, the interrupt line to the 6502 or the 6502.[1]
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References

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  1. ↑ 1.0 1.1 1.2 1.3 1.4 1.5 Atari Corp., 130XE Reference Manual (cover dated March 1990): parts list of 25 June 1985 (Q7 MPSA55 C014809; R91 2.7 kΩ; R128-131 100 Ω; C301-310 1000 pF C070121); production test diagnostic error codes 34 to 36; schematic diagram C070065 Rev. B (POKEY serial lines through R128-R131 to J1; PIA CA1, CA2, CB1, CB2; Q7; L10, L11; J1 pinout). Hosted here as Atari 130XE Parts List and Production Test Instructions.
  2. ↑ 2.00 2.01 2.02 2.03 2.04 2.05 2.06 2.07 2.08 2.09 2.10 2.11 2.12 2.13 2.14 2.15 2.16 2.17 Howard W. Sams & Co., Sams Computerfacts CC16, Atari Model 130XE: troubleshooting, "Cassette Save and Load" (PDF page 10, continued on PDF page 11 with the motor-control check); preliminary service checks ("Cassette") and disassembly instructions (PDF pages 19 to 21). Hosted here as Sams Computerfacts Atari 130XE.
  3. ↑ 130XE Personal Computer Field Service Manual, reproduction by JER/Eightbitter, November 2016, section 4, console assembly. Not an Atari publication. Hosted here as Atari 130XE Field Service Manual.


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